You're not missing anything, but it all comes down to what is the best fit for your needs, and it's not always about weight alone.

The same argument could be made about automobiles - if you have a 'large' car, you clearly know a 'subcompact' is much smaller - but what would be the argument for a 'midsize' car? Why do so many pick one, when they're not THAT much smaller than a large car? It's usually because it's a size that just fits the person's needs the best - not too large, not too small.

I can only give you MY experience with mirrorless, and why I love having one as a second body to my DSLR. First off, my DSLR is not one of the super-lightweight compact models - it's solidly a middle size body...which I like. The weight and heft is good, the grip is solid, and the balance is very nice with large lenses. But come summer, when it's near 100 degrees, and humidity to match, and I'm outdoors walking 10 miles with a heavy camera rig, I start wishing I could shed a few Lbs from my gear. And that's when the mirrorless cam comes to play - it's smaller and lighter - not THAT much lighter, but enough to matter. I can opt to use a lesser lens (rather than an F4 prime, I'll use an F4-6.3 zoom) and knock the overall size and weight down significantly (my DSLR birding rig is 9Lbs, my mirrorless rig is 3Lbs). And it's not just about WEIGHT as I mentioned - it's also about BULK - the mirrorless body is much thinner, much less bulky - it can be carried several ways that make it more convenient - I can hang it comfortably from a belt clip, I can stick it under my arm by the lens barrel, etc. My model is a very thin body with a very thin lens registration, but with a pronounced grip, which still makes it very comfortable to carry long distances - I don't get on well with the gripless rectangle bodies that are harder to carry by the body.

And it's also about the modularity. While a mirrorless body with a big telephoto lens isn't saving much in the way of weight or size over a DSLR with the same lens, I have the option to knock the camera down to just the body and a super-small pancake lens, or compacting zoom lens. In those cases, the camera can be made as small as some P&S cameras, and even pocketable. And even considering additional lenses, because of the lack of bulk in the camera body itself, I can stick my mirrorless body and 3 smaller lenses in a tiny camera bag, occupying the exact same space that my DSLR would with one lens attached. I can make the mirrorless as small as a P&S, or expand it up to nearly DSLR size and use it with longer-reach telephotos, as the situation calls for. I consider it my 'midsize' car, or SUV. My DSLR is my brute - the big monster that can do any job, but some days I just want to take the smaller, less-fuss car, which can do the same job under most but the most extreme circumstances...and be a lot easier to park and maneuver. It's still bigger than a subcompact, but it's also much more capable than a subcompact...and that's why it's the right size for me.

I'm committed to both DSLRs and mirrorless. I still don't feel like a mirrorless is the best solution when I need to shoot with very long or very heavy lenses - the DSLR is just better balanced with those lenses for me...but I love mirrorless bodies' ability to save me some weight and a good bit of bulk when working with shorter lenses and primes, carry multiple lenses in the same bag as a single lens for my DSLR, and the ability to use as a backup birding rig when the weather is awful hot and humid and I need to go long distances with a lot less bulk and weight along for the ride.